The surface of human epidermis is composed of corneocytes, flat-shaped keratin proteins formed from keratinocytes which are the major component of the epidermis generated from the stratum basale by cell proliferation. The corneocytes function to maintain the smoothness and elasticity of the skin by binding to the double-chained lamellar structure of ceramides which are abundantly present in the stratum corneum. A ceramide is a sphingolipid composed of sphingosine and a fatty acid linked to sphingosine or phytosphingosine. A family of ceramides, which make up about 40% or more of the lipids present in cornified cells, is known to be essential in controlling the structural organization and functions of the stratum corneum, and they also have other vital physiological properties.
Ceramides protect the skin from internal/external stresses, e.g., by removing disrupted cells, and they become depleted with advancing age. As a consequence, water loss, exposure to the external stimuli such as ultraviolet or chemicals, and exfoliation of cornified cells take place, which causes the skin to become dry, cracked and fissured.
Such symptoms may be partially ameliorated by external application of ceramides. However, if a large amount of ceramides are used in a cosmetic formulation, instability and gelation of the formulation may occur.
The present inventors have unexpectedly found that a complex mixture composed of a pseudoceramide, a cholesterol, and a stearic acid is stable and has an improved water retention capacity, and that a composition containing the same has improved capabilities for skin moisturization and restoration of the interest ceramide functions.